This invention primarily relates to metal alkoxides and metal alkoxide solutions suitable for use as precursors in the production of metal oxide coatings on glass and similar substrates, to methods for the preparation of such metal alkoxide precursors and to the coatings and devices employing such coatings. The glass substrates of particular interest are those employed in large-area electrochromic xe2x80x98smart windowsxe2x80x99 that require a variety of nanocrystalline metal oxide coatings.
Unless otherwise made clear by the context:
xe2x80x98Metalxe2x80x99 refers to any of the conventional metals and also those xe2x80x98metalsxe2x80x99 or xe2x80x98metalloidsxe2x80x99 of Groups III-A, IV-A and V-A of the Periodic Table of the elements, such as silicon, boron, aluminum, tin, antimony and the like.
xe2x80x98Metal alkoxidexe2x80x99 refers to any metal compound derived from an alcohol or alcohol-containing organic moiety wherein the hydroxylic hydrogen of at least one hydroxyl group has been replaced by a metal, and thus includes alkali and alkali earth metal alkoxides.
xe2x80x98Metal halidesxe2x80x99 includes oxymetal halides.
The metal alkoxides and alkoxide precursor solutions of this invention have particular use in the production of nanocrystalline metal oxide films on large-area glass panels. The films are of high quality, transparent and suitable for use in electrochromic devices. They have valuable application in other areas of industry and technology, such as in catalysts and electronic devices.
Metal alkoxides are important in the formation of thin nanocrystalline transparent layers of metal oxides on glass substrates by sol-gel techniques. Typically, the metal alkoxide is applied in a liquid solvent (precursor solution) to a substrate such as glass by dip-coating or spin-coating. The solvent is removed by evaporation and the metal alkoxide is exposed to water vapour to enable hydrolysis and condensation to produce a metal oxide and/or hydroxide sol-gel and alcohol. The coated substrate is then baked at moderate temperatures to remove residual alcohol and form the desired nanocrystalline metal oxide thin layer.
The production of alkoxides from metal halidesxe2x80x94especially metal chloridesxe2x80x94for use in electrochromic devices is well known, but the methods are generally tedious and can result in low yields because of the problem posed by the removal of the hydrogen halide by-product. Whereas the reaction of metal chlorides with any of the common alcohols is mildly exothermic and proceeds without difficulty, the standard method of removing by-product HCl (Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P.; Metal Alkoxides, Academic Press, London, 1978) is laborious and involves the use of anhydrous agents in non-aqueous solvents. The HCl by-product is usually allowed to react with a base such as ammonia, alkyl amines, pyridine or sodium alkoxide to produce chloride salts that are insoluble and precipitate from the solvent used. The above textbook reports the use of ammonia for the preparation of alkoxides from the metal halides of Si, Ge, Ti, Zr, Hf, Nb, Ta, Fe, Sb, V, Ce, U and Th. It also reports the use of sodium alkoxide for the preparation of the corresponding metal alkoxides from the metal halides of Ga, ln, Si, Ge, Sn, Fe, As, Sb, Bi, Ti, Th, U, Se, Te, W, lanthanides, Ni and Cr.
These methods, however, are cumbersome and suffer from several disadvantages. The fine precipitates (e.g. NH4Cl, NaCl) are impractical to filter and the products are obtained only after several prolonged steps of settling, decantation and washing with excess solvent to obtain the maximum yield. Further, in some cases (e.g. Sn, W) the products are often contaminated by the presence of varying amounts of chloride and sodium ions and also NH3 or its derivatives (Bradley, D. C., Caldwell, E. V., and Wardlaw W., Journal of Chemical Society, 1957, 4775; also, vide infra, Example 1). Washing the non-aqueous solution with water to remove chlorides is unacceptable because the metal alkoxides would be rapidly transformed to metal hydroxides and alcohol.
In Japanese patent [Sho 61-36292 (1986)] the following reaction between tungsten hexachloride and n-butanol was described in which part of the HCl was removed under reflux in CCl4 and the remainder with gaseous NH3 as a precipitate of NH4Cl after the addition of benzene:
WCl6+ROH+NH3xe2x86x92Oxe2x95x90W(OR)4+RCl+nNH4Cl+(5xe2x88x92n)HCl
After three extractions with benzene a yield of 85% of tungsten (Vl) oxo-tetra-n-butoxide [Oxe2x95x90W(OBun)4] was obtained. The method, however, has the same drawbacks as noted above.
In particular, we have found that, during the preparation of tungsten (Vl) oxo-tetra-alkoxide, [WO(OR)4], from WOCl4, alcohol and ammonia in n-pentane, an insoluble tungsten-containing compound often coprecipitates with ammonium chloride, making the extraction of the product extremely difficult. Reasons for this behaviour are unclear, but insoluble tungsten material could be due to dimer formation or some other incompletely understood interaction of NH3 and/or NH4Cl with tungsten alkoxide in hydrocarbon solvent. Excess ammonia can be added to dissolve the precipitated tungsten compound, but we have found that the final tungsten oxide obtained from such precursors is unsuitable as a film for electrochromic applications since the reversibility of the colouration-bleaching cycle is inadequate. This is still the case even when no significant amounts of chloride ions and ammonia-derived impurities were detectable in the alkoxide. We believe that the behaviour of WO3 in the film prepared by this route is heavily dependent upon the structure of the precursor tungsten alkoxide, which in turn is influenced by the NH3 concentration during its preparation. Furthermore, we have found that alcoholic solutions of metal alkoxides prepared by the NH3 route are often unstable, resulting in the precipitation of insoluble metal-containing material over time. This means that a single batch of alkoxide produces variable quality coatings.
Similar difficulties with the removal of chloride are encountered in the production of tin alkoxides for use in glass coatings and are addressed in U.S. Pat. No. 4,731,461 (1988). This patent teaches the use of ammonia as described by Bradley et al. as the first step of a two step process in which the product of the first step is treated with a metal amide or a metal alkoxide and additional alcohol. This results in the precipitation of a metal halide salt that can then be removed by filtration. With other metals of interest, particularly tungsten, the two step process taught by U.S. Pat. No. 4,731,461 is not satisfactory. As explained above, the first step is cumbersome, has a poor yield and generates little understood side products that render the resultant alkoxide material unsuitable for electrochromic purposes.
Electrochromic Coatings from Solutions of Metal Chlorides and Alcohols
A number of prior-art patents disclose the production and use of metal alkoxides for use in electrochromic coatings but do not address the problem of chloride removal. For example, in U.S. Pat. No. 4,347,265 (1982) tungsten (Vl) chloride (WCl6) is dissolved in an organic solvent such as methanol, isobutanol, ethanol, or acetic anhydride and it is implied that the resultant solution is applied to the substrate without further treatment and then baked to form the desired electrochromic layer. Similarly, U.S. Pat. Nos. 4,996,083 (1991) and 4,855,161 (1989) disclose the preparation of electrochromic coating solutions from anhydrous transition metal halides, preferably chlorides, such as tungsten chloride, and lower carbon, anhydrous alcohols, but no manner of removing contaminating chloride is disclosed. While U.S. Pat. No. 5,659,417 (1997) discloses the use of tungsten and molybdenum alkoxides in alcohol solutions, the method of preparing the alkoxides from metal chlorides, is that of U.S. Pat. No. 4,996,083 outlined above.
Our experience has shown that satisfactory coatings can be made in rare instances by the methods of the just-mentioned patents without removing the chloride impurities. The precursor solutions, however, are unstable and the coatings are prone to discolouration, crazing and, if electrochromic, have poor reversibility and bleaching qualities.
Reactions of Anhydrous Metal Chlorides with Epoxides
The reactions between epoxides and a number of anhydrous metal chlorides have been reported in earlier texts (Heterocyclic Compounds with Three- and Four-membered Rings, Part One, Arnold Weissberger, Editor, Interscience Publishers, 1964, pages 446-451). The mechanisms of reactions leading to the progressive formation of haloalkoxy metal derivatives as well as those leading to the hydrolysed products from these derivatives were discussed. It is known, also, that certain inorganic halides such as AlCl3, SnCl4, BF3 can react vigorously even explosively with epoxides to form tarry or polymerised products. To circumvent these difficulties inert solvent and procedures such as cooling and controlled addition of the epoxides were introduced.
In this context, U.S. Pat. No. b 2,709,174(1955) discloses the careful addition of propylene oxide to anhydrous titanium chloride in aliphatic or aromatic hydrocarbon solvents to avoid side reactions. Nevertheless, the isolated haloalkoxide metal derivatives were reported to be dark brown or black viscous materials probably because of thermal decomposition. U.S. Pat. No. 2,706,181(1955) and U.S. Pat. No. 2,706,189 (1955) disclose the addition of propylene oxide to anhydrous ferric chloride with the preferred solvent being diethyl ether rather than hexane because of the viscous nature of the final product. The materials isolated from this reaction are effective catalysts for the conversion of olefin oxides to solid polymers. Their empirical formula corresponds to FeX3(C3H6O)n where X is a halogen atom and n has a value from 0.5 to 3.0. The halogen atoms were considered to be part inorganically and part organically bound.
To avoid the disadvantages of readily hydrolysed alkoxy bonds such as those present in Ti(OR)4 where R is an organic group,the authors of U.S. Pat. No. 3,931,260(1976) and U.S. Pat. No. 3,934,068(1976) have disclosed the use of organo-metallic compounds containing more complex beta-haloalkoxy groups with 10 to 25 carbon atoms that were purported to be more resistant to hydrolysis. The compounds were prepared by the reaction of metal halides such as SiCl4 and TiCl4with one or more higher-molecular-weight epoxides usually in the presence of hydrocarbon solvents. For the purpose of coating glass fiber surfaces to impart abrasion resistance, solutions were made by dissolving the compounds in diacetone alcohol (4-hydroxy-4-methylpentan-2-one) or acetone. Evidently,these polar solvents are convenient for dissolving the higher-molecular-weight organometallic compounds, since the unreactive tertiary alcohol and ketone functional groups would not be able to cleave the metal-oxygen bond formed from primary or secondary haloalkoxy groups. It was asserted, also, that abrasion resistance could be imparted to the coated glass surface by the thermal decomposition of these products to metal oxides, but examples were not given. The metal alkoxides and their derivatives containing up to 18 carbons atoms mentioned in these patents are not suited for producing high-quality metal oxide coatings on glass surfaces, however, because of the likely risk of contamination by carbon from the involatile organic material present and such use was not disclosed.
Since the order of interchangeability of alkoxy groups in alcoholysis reactions (D. C. Bradley, R. C. Mehrotra and D. P. Gaur in Metal Alkoxides,Academic Press, 1978, page 28) is: MeO greater than EtO greater than PriO greater than ButO the solvolysis of haloalkoxy boron derivatives by methanol was used effectively as an analytical tool by earlier workers (J. D. Edwards, W. Gerrard, and M. F. Lappert, J. Chem. Soc. 348,. 1957).The structure of the borate esters was able to be deduced from a study of the solvolysis products. The borate esters were converted to methyl borate and haloalcohols that then could be isolated and identified. The trischloroalkyl borates (RO)3 B were prepared from the direct addition of the neat propene oxide to the boron trichloride at xe2x88x9280xc2x0 C. and after allowing the mixture to attain room temperature, treated with methanol. No mention of mixed metal alkoxides/haloalkoxides was made.
Methanol, furthermore, would be an unsuitable solvent in the present invention because most metal methoxides are involatile and form insoluble oligomeric species(see Metal Alkoxides, idem, page 46) from which it would be difficult to make dip-coat solutions. Hence, the present disclosure in which alcohols other than methanol exchange with haloalkoxy groups to form mixed metal alkoxy/haloalkoxy compounds is distinct from the prior art.
Reactions of Hydrated Metal Chlorides with Epoxides
The following patents describe the interaction of hydrated metal chlorides and other hydrated metal compounds with epoxides.
U.S. Pat. No. 3,458,306 (1969) discloses the addition of epoxy compounds to hydrated metal chlorides of nickel and aluminum dissolved in an alcohol to produce a metal hydroxide gel. The gel is first heated in an oxidising atmosphere to remove the remaining organics and is then heated in a reducing hydrogen atmosphere to reduce the hydroxide to the metal powder. However, the method cannot produce intermediate alkoxides because of the presence of the water of crystallisation in the hydrated metal chloride. Similarly, U.S. Pat. No. 4,551,358 (1985) discloses a method for the preparation of nickel oxide electrodes by dissolving hydrated cobalt and/or nickel chloride (eg. NiCl2.6H2O) in an alcohol, optionally with additional water. After applying this solution to a porous nickel substrate and allowing the epoxy compound to contact the impregnated substrate, the volatile by-products are removed by baking. As in the last mentioned patent, the formation of alkoxides for producing metal oxide coatings on glass is prevented by the presence of water in the solution derived from the water of crystallisation and such use was not disclosed.
European Patent No. 369,979 (1989) discloses a way of making fine spherical powders of amorphous hydrated zirconium oxide. In the given example, a process was described in which zirconium oxychloride octahydrate (ZrOCl2.8H2O) was dissolved in 2-methoxyethanol at 60xc2x0 C. and it was claimed that water was removed as an azeotropic mixture of water and 2-methoxyethanol to produce an essentially anhydrous zirconium salt. It was further claimed that to this zirconium salt was added n-propanol followed by the successive additions of propylene oxide and triethylamine to afford an organic solution of zirconium propoxide. This intermediate zirconium compound was then treated with a mixture of water, oleic acid and n-propanol to effect the hydrolysis and to produce spherical particles of zirconium oxide.
2-Methoxyethanol, a reactive chelating alcohol (Poncelet O., Hubert-Pfalzgraf L. G., Daran J. C. and Astier R., J. Chem. Soc., Chem. Commun., 1846, 1989) was used for dehydration but it was indicated to be an inert solvent merely used to remove water as an azeotropic mixture. To this mixture was then added n-propanol, propylene oxide and triethylamine. The propylene oxide was indicated to act only as an acid scavenger and no explanation for the use of triethylamine was proffered.
Although the reaction details of the dehydration process and structure of the final product remain speculative, the presence of water and a chelating alcohol during the reactive drying process would most likely produce a complex mixture of aggregated zirconium species in which oxo bridges were formed together with chloride and possibly some hydroxy and 2-methoxyethoxy groups remaining associated with the metal ion. Support for this assertion is strengthen by the structure of the initial zirconyl species in question, which is formulated as [Zr4(OH)8 16H2O]+8. The zirconium atoms are arranged in a square and linked along each edge of the square by a pair of OH groupsxe2x80x94one above and the other below the zirconium plane (Clearfield A. and Vaughan P. A., Acta Crystallogr., 1956, 9, 555). There are four additional water molecules bound to each Zr atom and the remaining 12 water molecules and 8 chloride ions form a matrix holding the zirconyl complexes together. Also, the zirconyl complex ion has a propensity to form mixtures of aggregated zirconium species under certain reaction conditions (Singhal A., Toth L. M., Lim J. S. and Affholter K., J. Am. Chem. Soc., 1996, 118, 11529).
Hence, the above-mentioned method, cannot produce an anhydrous metal chloride because of the interaction of a chelating alcohol with a hydrated zirconyl chloride and the disclosure, therefore, is considered to be distinct from that of the present invention. Also, as stated earlier, the presence of ammonia- or alkylamine-derived products gives inferior metal oxide coatings on glass substrates for electrochromic purposes and hence, the above-mentioned process was judged to be unsuitable.
Summary of the Background to the Invention
Since the removal of hydrogen chloride by neutralisation with ammonia during metal alkoxide preparation often gave rise to reaction conditions that led to undesirable side reactions, we have sought to overcome these disadvantages by seeking alternative methods.
We have found that the addition of an epoxide to anhydrous metal halides in the presence of an alcohol results in the removal of the halogen atoms by establishing metal-haloalkoxy bonds as well as haloalcohols. The progressive replacement of halogen atoms by alkoxy and haloalkoxy groups in the metal halides eventually leads to the formation of metal alkoxy/haloalkoxy mixtures. Unlike prior-art methods of allowing epoxides to react directly with anhydrous metal halides,we have found, that if an epoxide is added to an admixture of metal chloride, alcohol and first reaction products,the usually highly exothermic reaction is more easily controlled and thus thermal decomposition is avoided. This procedure ensures, in most cases the efficient substitution of the halide atoms of the metal halides by alkoxy/haloalkoxy groups without tarry products being formed due to the polymerisation of the epoxides. The displaced volatile haloalcohols, excess alcohol and solvent can be readily removed by evaporation from the metal alkoxide product to leave a residue that can be easily re-dissolved in a suitable solvent. This method allows for the effective removal of the halide ions from the metal halide without causing complicated side reactions like those associated with the use of NH3 and alkylamines. The use of such alkoxide precursors results in superior nanocrystalline coatings of metal oxides on glass substrates. We have also found in the case of WCl6 and VOCl3 that alcoholic solutions of their respective metal chloroalkoxides prepared without the removal of the chloroalcohols perform surprisingly well in forming oxide coatings on glass. It appears that any organic chlorine-containing compounds vaporise from the humidified film during drying and/or baking, leaving the properties of the metal oxide coating unaffected.
The general objective of the present invention is to provide methods for the preparation of metal alkoxides that will avoid one or more of the disadvantages associated with the prior art. More particularly, but not essentially, a further objective of the invention is to provide alkoxide coating precursor solutions suitable for use in forming coatings of metal oxides on large-area substrates, such as glass, by the sol-gel technique.
From one aspect, the present invention involves the reaction of anhydrous metal halides with alcohols so that the presencexe2x80x94or the additionxe2x80x94of an epoxide will result in the formation of a complex mixture of metal alkoxides with alkoxy and haloalkoxy groups attached to the metal. The alcohol can be added to the anhydrous metal halide before the addition of the epoxide or added simultaneously with the epoxide as an admixture. The alcohols can be added in excess or in an amount less than that required stoichiometrically to substitute all chlorine atoms. The molar ratio of alcohol to halogen atom can vary in the range from 10:1 to 0.01:1 but preferably in the range 2:1 to 0.1:1. Any displaced haloalcohols, excess alcohol and inert solvent are readily removed by evaporation or distillation. Exothermic reactions are carried out in the presence of inert hydrocarbon or polar solvents such as pentane, hexane, tetrahydrofuran or diethyl ether to dissipate the heat and lessen thermal decomposition. The preferred organic solvent for most metal chlorides is n-pentane or hexane. With some metal chlorides, for example, ZnCl2, TaBr5, (vide infra, Examples 22, 26) the reaction can be carried out conveniently without solvent and in this case the molar ratio of alcohol to halogen atom can vary in the range 100:1 to 10:1.
From another aspect, the present invention discloses the unexpected result regarding the usefulness of these mixed metal alkoxides as precursors suitable for laying metal-oxide coatings on large-area substrates by the sol-gel techniques. Hydrolyses and condensation reactions of metal alkoxides with alkoxy and/or haloalkoxy groups, free of the influence of alkylamine- or ammonia-derived products, have proven to give superior metal oxide films compared with those prepared by the conventional route. Electrochromic films (eg. WO3) formed from such precursors have been found to have good reversibility and long working lives.
From a further aspect, the present invention discloses the fact that the amount of by-products resulting from adverse reactions such as thermal decomposition or oligomerisation of the epoxides in the presence of metal halides is small, if any under the present conditions Despite the presence of metal alkoxide mixtures and the possibility of by-products being formed, metal oxides films obtained by these procedures are superior in performance to those from prior-art methods.
A typical procedure for forming a transparent metal oxide coating on glass will therefore be: (i) suspend or dissolve the anhydrous halide(s) of the selected metal(s) in an anhydrous organic solvent, (ii) add an alcohol and then an epoxide or a mixture of an alcohol and an epoxide at a rate to control the reaction which forms the metal alkoxide/haloalkoxide mixtures, (iii) remove the volatile components from the reaction product by evaporation or distillation to thereby remove any organic halogen-containing species, (iv) dissolve the residue in an alcohol to form a precursor solution, (v) apply the precursor solution to the glass as thin film, (vi) dry the film releasing any excess alcohol, solvent or residual volatile components, solidifying the coating, (vii) allow the film to adsorb moisture and hydrolyse to form a sol-gel, and (viii) bake the glass and the film to form the desired transparent metal oxide coating.
If the displaced haloalcohols and other volatile components are removed by distillation, the resulting residue is readily re-dissolved in alcohol to give stable precursor solutions from which thin, transparent and substantially defect-free nanocrystalline metal oxide films can be formed on glass substrates by the simple dip-coating, hydrolysis and firing steps known in the art. Preferably, a minor proportion of water is included in the precursor solutions (for example, by inclusion in ethanol) to improve its physical properties.
Epoxides such as ethylene oxide, propylene oxide and butylene oxide can interact initially with either the anhydrous metal halides or with the hydrogen halides generated from the reaction of anhydrous metal halides with alcohol. The addition reaction of hydrogen halides to epoxides results in the formation of halogenated alcohols which in turn can also displace halide anions from the initial metal halide, oxymetal halide or its intermediates to form metal haloalkoxy derivatives and a further molecule of hydrogen halide. Epoxides that react directly with anhydrous metal halides would form halo metal haloalkoxides intermediates. The final distribution of metal alkoxide species, however, would depend upon the reaction rates of the competing processes as shown below. The molar ratio of epoxide to halogen atom can vary in the range from 1:1 to10:1 but preferably at least 2:1. 
where: xe2x80x98epoxidexe2x80x99 denotes preferably ethylene oxide, propylene oxide or butylene oxide; 0 less than n less than 6; 0xe2x89xa6xxe2x89xa64; 0xe2x89xa6yxe2x89xa64; x+yxe2x89xa64; 0 less than m less than 6; m+nxe2x89xa66; M is a metal or metal oxide; R is an alkyl group preferably with 2 to 10 carbon atoms and R1 is a chloroalkyl group resulting from the addition of HCl to the corresponding epoxide or from the direct reaction of an epoxide with metal halide or halo metal alkoxide/haloalkoxide intermediates.
As already noted, it is envisaged that the epoxides could react directly with the metal halide in a solvent to displace and capture halide ions in a process forming metal haloalkoxide derivatives. After removing solvent and replacing with alcohol, dip-coat solutions are formed containing metal alkoxide mixtures with alkoxy and haloalkoxy groups and displaced haloalcohols. However, it must be noted that large amounts of haloalcohol fumes given off by films formed from such dip-coat precursor solutions are toxic and would require suitable safety measures during the drying and baking stages. Prior-art methods that have employed sodium alkoxide or ammonia to remove by-product HCl were often contaminated by NaCl or NH3-derived residues, but this problem is avoided by the methods of the present invention.
The method of the invention can be adapted to form alkoxides from a variety of halides, alcohols and epoxy compounds and metal elements in various oxidation states, though halides other than chlorides (eg. fluorides, bromides and iodides) are generally less satisfactory. Metal chlorides which can be used to produce alkoxides by the method of this invention include: WCl6, WOCl4, WCl4, MoOCl4, VCl5, VOCl3, VCl3, NbCl5, TaCl5, TiCl4, ZrCl4, ZrOCl2, IrCl3, FeCl2, FeCl3, CoCl2, NiCl2, CuCl2, ZnCl2, CdCl2, BCl3, AlCl3, GaCl3, InCl3, TICl, SiCl4, GeCl4SnCl4,PbCl4, SbCl5, YCl3, and lanthanide chlorides. All traces of adventious moisture and chemically combined water should be absent from the metal chlorides. Preferably the metal chlorides are heated under reflux with appropriate reagent such as thionylchloride and the excess is removed by careful distillation at atmospheric pressure and finally under vacuum.
The alcohols can be straight-, branched- or cyclic-alcohols, preferably, containing 2 to 10 carbon atoms, most preferably 2 to 8 carbon atoms. Similarly, a wide variety of epoxides may be used with the lower carbon epoxides such as ethylene oxide, propylene oxide or butylene oxide being preferred. Even polymers or other high molecular weight materials containing epoxide groups, which are insoluble in the reaction medium eg. bisphenol-A-diglycidylether, triglycidylisocyanurate, glycidyl 3-(pentadecadienyl) phenylether, poly[(phenyl glycidylether)-co-formaldehydel] may be used for ease of separation from the other volatile reaction products in certain cases.